Coordinate action of a proximal homeoprotein binding site and a distal sequence confers the Ultrabithorax expression...

Spatially regulated expression of the homeotic gene Ultrabithorax (Ubx) in the visceral mesoderm can be mimicked in transformed Drosophila embryos by expression of a Ubx--beta-galactosidase fusion gene. Here we show that a proximal homeoprotien binding sequence downstream of the Ubx transcription start site, the B element, is required for this pattern. A distal upstream Ubx sequence, but not the B element, is sufficient to confer the pattern if linked to an hsp70 TATA box in a heterologous construct. The pattern in this case requires Ubx function, like endogenous Ubx expression in the visceral mesoderm, suggesting that the distal upstream sequence contains an important target sequence for autoregulation. We propose that the B element, in the context of the Ubx promoter, functions to mediate enhancer function of the distal sequence. Thus, the visceral mesoderm pattern requires coordinate action of a proximal and a distal regulatory element.